Amazon started selling diapers for Procter & Gamble through product pages inside Facebook late last week.

The Pampers fan page on Facebook has a "shop now" tab on it. If you want to buy pampers, just click on it, use your Amazon account, and boom, you've bought diapers through Facebook.

It sounds like a small, somewhat obvious step, for Facebook and fan pages. But, Lazard Capital Markets analyst Colin Sebastian thinks this will be the next major revenue opportunity for Facebook down the road.

For now, Facebook is not collecting any fee from Amazon or Pampers. However, Sebastian points out Facebook didn't make any money from Zynga for years. Eventually, it established its credits system and started taking a cut of Zynga's revenue.

Same thing will happen with ecommerce. As more companies choose to sell goods through fan/product pages, Facebook will be poised to collect on each transaction. Facebook will also benefit from an increase in ads, as companies promote their product pages.

This is another example of Facebook diversifying its revenue streams. Here's all the revenue streams Facebook is pursuing, from a story we wrote in early September:

Search ads – All these "Like" button are building a hierarchy of Web pages, similar to Google's Pagerank.

Facebook Credits in games – Thanks to hugely popular games like Zynga's Farmville, this could already be a third of Facebook's revenues in 2010.

Pay with Facebook on third-party ecommerce sites – If your credit card is already in the system, why not just click "Pay With Facebook" instead of going through the hassle of a check-out process?

Local coupons – Facebook just launched a "check-in" feature called Places. Groupon is showing that local businesses will buy coupons if they will draw a crowd.

Brand advertising on Facebook.com – Brands like Facebook ads because they drive traffic to Facebook pages. If user "Likes" a Facebook page, that page owner can them spam them almost at will.

Brand advertising off Facebook.com – Almost 100 million Facebook users log-in to third-party sites using their Facebook IDs through the program that used to be called Facebook Connect. Facebook knows more about these visitors to third-party sites then the third-parties themselves. This puts Facebook in a position to either sell ads for those third-party sites, or to sell anonymized data to those third-parties sites, so they can themselves sell better-targeted ads.

Disclosure: Jeff Bezos is an investor in Business Insider through his
personal investment company Bezos Expeditions.